It is known to provide a collector for collecting particulates in an exhaust pipe of an internal combustion engine. The collector reduces particulates in the exhaust gas flowing in the exhaust pipe.
As shown in FIG. 13A, the collector is made of a porous partition wall 402 with many exhaust passages 401 extending therethrough. The collector adsorbs and collects particulates P as the exhaust gas G flows through the exhaust passages 401. As the particulates P are collected, pressure loss through the exhaust pipe increases and engine efficiency decreases. Thus, when the amount of deposit reaches a specified amount, the particulates P in the collector are burned to reduce the particulates P and recondition the collector. For instance, the collector includes oxidation catalysts 403, and HC is supplied to the collector to raise the temperature of the collector by means of a catalytic reaction of the HC.
To estimate the amount of particulates P collected, a differential pressure expression is used, which is based on a differential pressure detected across the collector and a history expression for estimating the amount of deposited particulates P. To estimate the amount of deposited particulates P in an estimation technique using a history expression, it has been proposed to compute the amount of decrease of particulates P on the basis of temperature in the collector. (See, for example, Japanese Patent Publication No. 2001-280118A.)
Estimation of the amount of particulates can be inaccurate depending on the age of the collector. For instance, when the collector is new as shown in FIG. 13A, the particulates P directly contact the oxidation catalysts 403. Hence, approximately all of the particulates P are reduced during reconditioning of the collector.
However, when the collector has been used for a significant time as shown in FIG. 13B, ash A of flame-retardant fine particles are deposited on the upstream side of the partition wall 402 and the oxidation catalysts 403 are substantially covered. As such, there is less contact between the particulates P and the oxidation catalysts 403, which leads to computational error of the amount of decrease of particulates P during reconditioning. Thus, the estimated amount of deposited particulates P may be inaccurate for an estimation technique that utilizes a history expression.